Information recording devices such as magnetic disks are always required to have a larger recording capacity and to attain a reduction in access time such as disk rotational delay. One possible means for satisfying the latter requirement is to heighten the rotational speed of a medium.
However, the media comprising a substrate currently in use are weighed down by themselves and resonate considerably at an increased rotational speed. Eventually, the surface of such a medium comes into contact with the head to cause an error or crushing. It is therefore impossible to narrow the gap between the magnetic disk head and the recording medium to or below a certain level, and this constitutes a serious obstacle to an increase in recording capacity.
For reducing the bending of a substrate medium and diminishing the resonance of the medium being rotated, it is necessary to heighten both the modulus of elasticity (Young's modulus) of the substrate of the medium and the rigidity thereof which is the value obtained by dividing the modulus of elasticity by the specific gravity.
The aluminum alloy which has been most commonly used as the substrates of magnetic disks has a modulus of elasticity of 71 GPa and a rigidity of 26 GPa.multidot.cm.sup.3 /g. This conventional substrate material, having such properties, hardly copes with the trend toward higher rotational speeds of 10,000 rpm and above. In addition, it has become necessary to increase the thickness of substrates made of the above material, although this goes against the current trend toward thickness reduction in disk substrates for device miniaturization.
In contrast, substrates made of a tempered glass are superior to the aluminum substrate in both modulus of elasticity and specific gravity. For example, a glass substrate obtained by subjecting a commercially available soda-lime glass to ion exchange in a molten potassium salt is on the market. This substrate has a modulus of elasticity of 72 GPa and a rigidity of 29 GPa.multidot.cm.sup.3 /g.
Also known besides the above one is a glass substrate obtained by tempering commercially available Corning 0317. Although this substrate has a modulus of elasticity of 72 GPa and a rigidity of 29 GPa.multidot.cm.sup.3 /g, these properties are still insufficient.
A high-rigidity substrate for information recording media which is made of a material other than tempered glasses is on the market. This substrate comprises a crystallized glass having a modulus of elasticity of 90 GPa and a rigidity of 38 GPa.multidot.cm.sup.3 /g. However, this substrate, after polishing, inevitably has residual crystal grains projecting from the surface because of the nature of the production process in which crystals are precipitated inside. Namely, this crystallized-glass substrate has a drawback that it is inferior in surface smoothness to the substrates made of a tempered glass.
Consequently, in view of the expected future trend toward even higher rotational speeds in information recording devices and smaller thickness in disk substrates, there is a desire for a glass composition which has further improved properties, i.e., which has a high Young's modulus and a high rigidity, can be easily tempered, and gives a substrate having high surface smoothness through polishing.